In a semiconductor device, a photodiode can convert received light into a current. It is widely used as light receiving element for optical pickup devices incorporated in CD players, DVD players, or other optical disc devices. The photodiode is constituted with a pn junction semiconductor. The depletion layer is widened by applying an inverse bias to the pn junction to form a strong electric field. Electron-hole pairs are generated by the light mainly absorbed in the depletion layer. Attraction by the electric field causes the electrons to move toward the n type semiconductor region, while holes move toward the p type semiconductor region, and a current can be detected.
Among the types of said photodiode, there are PIN photodiodes, which have an I layer (p− layer or n− layer) containing an electroconductive impurity at a low concentration between the p layer and the n layer and in which the depletion layer can easily widen at a low voltage, and avalanche photodiodes, which have a region where avalanche collapse occurs.
Silicon can only perform photoelectric conversion of light with a wavelength in the range of 400-1100 nm. Since the energy of light with a wavelength longer than 1100 is lower than the energy gap (1.12 V) of silicon, no electron-hole pair can be formed. Also, light with a wavelength shorter than 400 nm can only generate electron-hole pairs near the surface of silicon. Since the electron-hole re-coupling rate near the surface of silicon is usually very high, the generated electron-hole pairs will become re-coupled very quickly and disappear before photocarriers are detected.
Currently, for example, light with a wavelength of 780 nm or of 650 nm is used as the light source incorporated into optical pickups of optical disc devices. Such light, however, can only reach a depth of about 20-40 μm from the silicon surface. On the other hand, the use of light with a wavelength of 405 nm in optical pickups for optical disc devices has been studied for practical applications realizing high-density recording. Light in this region can only reach a depth of about 0.6 μm from the silicon surface.
Consequently, for a photodiode for receiving light with a wavelength of 780 nm or light in the red region with a wavelength of 650 nm, the sensitivity, CR time constant, and other high-frequency characteristics can be improved by forming the depletion layer in the region at a depth of 20-40 μm from the surface of silicon and by increasing the impurity concentration in the p region and n region that constitute the pn junction to reduce the resistance.
However, in order to receive light with a wavelength in the blue region of 405 nm, for example, it is necessary to form the depletion layer in the region near the surface layer at a depth of about 0.6 μm from the silicon surface. The structure of the conventional photodiode makes it difficult to for a depletion layer near the surface since the high-concentration impurity region is present near the silicon surface layer. Also, if the concentration of the electroconductive impurity present near the surface is reduced to from the depletion layer, the CR time constant will be increased. As a result, the response speed is lowered, and the high-frequency characteristics deteriorate.
Patent Reference 1 discloses a photodiode with improved sensitivity caused by reducing re-coupling near the silicon surface by forming a semiconductor layer of a second electroconductivity type so that there is a part subdivided by a semiconductor layer of a first electroconductivity type in at least one cross section in the surface layer part of the semiconductor layer of the first electroconductivity type.    Patent Reference 1: Japanese Kokai Patent Application No. 2001-320075.
In the photodiode disclosed in said Patent Reference 1, since the semiconductor layer of the second electroconductivity type is subdivided by the semiconductor layer of the first electroconductivity type, deterioration of the high-frequency characteristics becomes inevitable.
The purpose of the present invention is to solve the aforementioned problem by providing a photodiode that exhibits high sensitivity even to light with a wavelength in the blue region while maintaining its high-frequency characteristics.